In a cab signal systems for railroad and mass and/or rapid transit operation, it is necessary to provide a suitable power supply for furnishing operating voltages to the electrical and electronic components and circuits. While there are many regulated power supplies in existance, these previous voltage sources were possessed of several shortcomings which made them unacceptable for use in car-carried cab signal apparatus. In some cases, the prior art power supplies were large, bulky, and heavy as well as very expensive to construct which detracted from their general acceptance in the transportation industry, particularly in mass and/or rapid transit systems. In addition, the reliability of some existing power supplies, mainly, switching types of regulated voltage supplies, does not fall within specification of the authoritative requirements. While linear and constant voltage transformer power supplies have an acceptable reliability, it has been found that such power supplies are not generally directly applicable for operation with a battery bus. In practice, these latter two types of power supplies must be preceded by a d.c. to a.c. inverter which results in the degradation of the reliability of the overall circuitry. Further, power supplies having a mean time between failures (MTBF) of nineteen thousand (19,000) hours have been considered to be an unacceptable rate of failure for cab signal equipment. Generally, the unacceptable MTBF rate was the result of the complexity and number of components used in previous switching regulator power supplies.